1. Field of the Invention
The present invention relates to pressure gauges and filling methods, and more particularly, to a method of filling a bourdon tube pressure gauge with a gelatinous material for medical and other applications. The process allows standard off-the-shelf bourdon tube pressure gauges to be completely filled with a gelatinous substance.
2. Background Information
Bourdon tube pressure gauges are used in numerous medical applications, for example to monitor the internal pressures of balloons used to dilate blood vessels in an angioplasty procedures. Applicant's assignee has developed a gel filled bourdon tube pressure gauge which has several advantages over prior art gauges which are filled with oil, saline solution or other liquids, for example. Prior art hollow bourdon tube pressure gauges are known to have several problems in medical applications. Oil filled bourdon tube gauges require an isolating diaphragm. However, these isolating diaphragms require a large surface area to counteract the diaphragm's inherent stiffness, and the large size can compromise the sensitivity and accuracy of the gauge. Saline solution is highly corrosive to most metals typically used in apparatus construction. Also, the saline solution could induce electrolysis inside the bourdon tube due to the dissimilar metals used in its fabrication. And, electrolysis can put heavy metal ions into solution with the saline working fluid. Finally, the presence of air bubbles or voids in the prior art filled gauges causes an inherent safety problem in that if the balloon breaks, compressed gas may push fluid down the catheter shaft, through the ruptured balloon and into the patient's vascular system.
Significant advantages of the gel filled tube gauge, particularly the silicone gel filled gauge, include:
a. air need not be purged from the gauge or gauge tube: PA1 b. the silicone gel in the gauge isolates and insulates contaminates from the working fluid; PA1 c. the silicone gel does not compromise accuracy or functioning of the gauge; PA1 d. the silicone gel is simple, compact, and does not increase the size of the gauge or the gauge system; PA1 e. the silicone gel is cost effective; PA1 f. the silicone gel is chemically inert and biocompatible; and PA1 g. the silicone gel reduces dynamic effects to the gauge and the gauge system due to sudden pressure changes. Nothwithstanding these advantages the process of filling the bourdon tube completely with a gelatinous material has proven to be a difficult task, particularly on a production volume basis. The bourdon tube must be completely filled with gel to avoid the formation of air bubbles or pockets which can detrimentally affect the performance and accuracy of the gauge. The filling process must also be accomplished without changing the calibration of the pressure gauge. Other factors that contribute to filling problems are that (1) there are a variety of designs of pressure gauges, including varying bourdon tube and fitting configurations, and (2) medical products manufacturers and OEM suppliers typically obtain bourdon tube pressure gauges from industrial suppliers and then modify the gauges to meet their particular requirements. These factors necessitate that gel filling process be able to be accomplished simply, reliably and economically by diverse users and on diverse product designs. PA1 a) evacuating the interior cavity to a vacuum level not greater than 50 millitorr; PA1 b) depositing a predetermined amount of a gelatinous substance in the interior cavity; and PA1 c) venting the interior cavity to remove the vacuum level, whereby increasing air pressure forces the deposited gelatinous substance to completely fill the enclosed space. PA1 a) providing a vacuum chamber comprising a preconditioning shelf, a turntable, a stationary gel reservoir with a transfer conduit and valve disposed above the turntable, a vent valve, and a plurality of individual filling funnels; PA1 b) placing a plurality of structures in the vacuum chamber on the turnable: PA1 c) placing a plurality of structures on the preconditioning shelf; PA1 d) connecting a filling runnel to each structure at its exterior orifice, the filling funnel being oriented vertically upright; PA1 d) filling the gel reservoir with a predetermined amount of gelatinous substance; PA1 f) evacuating the entire vacuum chamber to a vacuum level not greater than 50 millitorr; PA1 g) sequentially filling each gauge by rotating the turntable so that the filling funnels attached to individual structures are aligned below the gel reservoir and actuating the valve to allow gel to flow from the reservoir through the conduit and into the filling funnel; and PA1 h ) releasing the vacuum via the vent valve, whereby increasing air pressure forces the deposited gelatinous substance to completely fill the bourdon tube. The benefits of this invention will become clear from the following description by reference to the drawings.